Conventional air purifiers incorporated with an adsorbent bed to remove gaseous pollutants are usually short-lived and non-regenerative. That means regular replacement of the adsorbent bed is required. Obee et al. (U.S. Pat. No. 6,358,374) discloses an integrated air purification system including a catalyst bed, a UV source and a regenerable adsorbent bed to exert photocatalytic oxidation activity. It applies the concept of Langmuir-Hinshelwood kinetics to such a system so that the rate of oxidation is improved even in a relatively low level of gaseous contaminants. However, extra heat is required in the '374 system to release the captured contaminants from the regenerable adsorbent bed into a fixed volume of a compartment. Another disadvantage is that the oxidation rate resulting from the titanium dioxide-based catalyst in this system is limited by the species of contaminant, amount of each contaminant being adsorbed and the release rate of the adsorbed contaminants from adsorbent bed. It also cannot provide a continuous air purification system if the adsorbed contaminants reach the saturation level that would trigger the regeneration cycle of the adsorbent bed.
Bohlen et al. (US2011/0033346A1) also discloses an air cleaner channel in an air cleaner with a photocatalytic oxidation substrate being activated by UV-A illumination. The substrate includes three coatings of VOC decomposing catalyst, ozone decomposing catalyst and titanium oxide, respectively. Although the substrate has different coatings for different functions, the efficiency of photocatalytic oxidation of the gaseous pollutants and ozone decomposition is limited by the contacting surface of each coating with the corresponding gaseous pollutants or ozone. The airflow should pass through the three coatings of the substrate in order to remove the gaseous pollutant and excess ozone, and thus, there is a need in the art for a single multi-functional coating having at least photocatalytic oxidation and ozone decomposition activities and a large contacting surface area for air purification systems.